Hey, I have a similar problem. I found it though. The left hole on my transmission cross member mount is becoming elongated and shifts sometimes when I accelerate forwards or backwards. Have him drop the tranny cross member and take a look.
To do that easily, have him disconnect the two bolts from the tranny mount and then jack up the tranny. At this point you can remove the bolts for the cross member and check out the mounting holes.
I discovered the problem when I was putting in a new polyurethane mount.
I haven't heard the problem since I put in the new mount, but my old one had 160k miles on it.
------------------
351W 89 Mustang GT Convertible
[This message has been edited by jimberg (edited 04-28-2001).]